A number of instruments, materials, and equipment are used in restorative, endodontic, prosthetic, orthodontic, surgical, and cosmetic dental procedures. One such instrument is an aspirating syringe (also called a dental syringe or local anesthetic syringe). The aspirating syringe is used in dentistry to inject a local anesthetic. The aspirating syringe differs from most syringes in that it is designed to inject anesthetic from a single-use disposable carpule. The parts of an aspirating syringe consist of a threaded tip where the needle attaches, a barrel where the carpule is placed, a syringe rod (or plunger) with a harpoon attached that embeds itself into the silicone rubber stopper of the carpule, a finger grip, and a thumb ring. The harpoon allows the dentist to aspirate (draw back) the injection site to see if the needle tip is located in a blood vessel before injecting the anesthetic solution. Once the harpoon is engaged into the silicone rubber stopper of the anesthetic carpule, the dentist can apply inward or outward pressure on the stopper by exerting pressure on the thumb ring. Pulling the thumb ring outward also pulls the plunger outward, producing an aspirating effect; whereas pushing inward forces the anesthetic solution through the needle.
One problem of aspirating syringes is that sometimes the stopper gets stuck on the harpoon and pulls out of the carpule when removing the carpule from the syringe. This complicates the need to change the carpule when multiple injections are needed, and the removal of the carpule before sterilizations procedures. The clinician risks injury from the sharp harpoon as one tries to pry the stopper from the harpoon within the syringe barrel. One approach to solving this problem has been to reduce the number and/or size of the barbs on the syringe rod harpoon. However, this sometimes causes difficulty in aspiration due to the rod disengaging from the stopper and only reduces the incident.